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This is a common failure on microwaves and it means that one of your 3 door switches is failing. It's usually the bottom switch, but if you have a multimeter you can test the continuity through the switch terminals of each switch when you open and close the door (microwave is unplugged and the switch you're testing is unplugged) and if it doesn't changed from normally opened to closed or vice versa---the switch needs to be replaced. Don't test the switches by just pushing on them with your finger, make sure you open and close the door to test them.
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